During the Blacksheep tour of duty (Pappy Boyington's I mean) one of the squadron members tested the ammunition and discovered that against the Japanese aircraft the incendiary ammunition had a much higher chance of starting an aircraft fire. The .50 incendiary at the time was actually an AP round with an incendiary component. Tracer rounds were included in their belts to indicate their firing line, but it was the incendiary bullets that actually lit the fires. And no, they were not lighting the vapor trail and I doubt that's what happened to you either. If you were firing your guns at the same time it is very likely you just did not hear the bullet that set you on fire.